In recent years, small-sized electronic devices represented by mobile terminals and the like have been widely spread and further down-sizing, weight saving and longer life are strongly demanded. To a market demand like this, developments of secondary batteries capable of obtaining, in particular, a smaller size, a lighter weight and a higher energy density have been forwarded. The secondary batteries have been studied to apply also to large-sized electronic devices represented by automobiles and power-storage systems represented by houses or the like without limiting to small-sized electronic devices.
Among these, a lithium ion secondary battery is highly expected because smaller size and higher capacity are easy to obtain and the energy density higher than that of a lead battery or a nickel-cadmium battery can be obtained.
The lithium ion secondary battery includes a positive electrode, a negative electrode, a separator, and an electrolytic solution. The negative electrode includes a negative electrode active material related to a charge/discharge reaction.
As the negative electrode active material, while a carbon-based active material is widely used, a further improvement in a battery capacity is demanded from recent market demand. As a factor for improving the battery capacity, it has been studied to use silicon as the negative electrode active material. This is because the silicon has a theoretical capacity (4199 mAh/g) of 10 times or more a theoretical capacity of graphite (372 mAh/g), and a great improvement in the battery capacity can be expected. A development of a silicon material as the negative electrode active material includes studies on not only a silicon simple substance but also on compounds represented by alloys, oxides or the like. Shapes of the active material have been studied from a standard coating type in a carbon material to an integrated type directly deposited on a current collector.
However, when the silicon is used as a main raw material as the negative electrode active material, since a particle of negative electrode active material expands and contracts during charge/discharge, cracks are likely to occur mainly in the neighborhood of a superficial layer of the particles of negative electrode active material. Furthermore, an ionic substance is generated inside the active material, and the particles of negative electrode active material are likely to be cracked. When the superficial layer of the negative electrode active material is cracked, a new surface is generated thereby, and a reaction area of the active material increases. At this time, the electrolytic solution is consumed since a decomposition reaction of an electrolytic solution occurs on the new surface and a film that is a decomposition product of the electrolytic solution is formed on the new surface. Therefore, the cycle characteristics become easily degraded.
Until now, in order to improve an initial efficiency and cycle characteristics of a battery, negative electrode materials for lithium ion secondary batteries having the silicon material as a main material and electrode structures thereof have been variously studied.
Specifically, in order to obtain excellent cycle characteristics and high safety, silicon and amorphous silicon dioxide are simultaneously deposited by using a vapor phase method (see, for example, Patent Document 1 below). Furthermore, in order to obtain high battery capacity and safety, a carbon material (an electronically conductive material) is provided on a superficial layer of particles of silicon oxide (see, for example, Patent Document 2 below). Furthermore, in order to improve the cycle characteristics and to obtain high input/output characteristics, an active material containing silicon and oxygen is prepared and an active material layer having a high oxygen ratio in the neighborhood of a current collector is formed (see, for example, Patent Document 3 below). Still furthermore, in order to improve the cycle characteristics, oxygen is contained in a silicon active material such that an average oxygen content is 40 at % or less (at % expresses an atomic composition percentage), and an oxygen content is high in a place close to a current collector (see, for example, Patent Document 4 below).
Furthermore, in order to improve a first time charge/discharge efficiency, a nano composite containing a Si phase, SiO2 and MyO metal oxide is used (see, for example, Patent Document 5, below). Still furthermore, in order to improve the first time charge/discharge efficiency, a Li-containing substance is added to the negative electrode, and pre-doping where the Li-containing substance is decomposed at a place where a negative electrode potential is high to return the Li to a positive electrode, is performed (see, for example, Patent Document 6 below).
Furthermore, in order to improve the cycle characteristics, SiOx (0.8≤x≤1.5, a particle size range=1 μm to 50 μm) and a carbon material are mixed and sintered at a high temperature (see, for example, Patent Document 7, below). Furthermore, in order to improve the cycle characteristics, an active material is controlled in the range such that a molar ratio of oxygen to silicon in a negative electrode active material is from 0.1 to 1.2, and, a difference of the maximum value and the minimum value of the molar ratio of an oxygen amount to a silicon amount in the neighborhood of an interface of the active material and a current collector is 0.4 or less (see, for example, Patent Document 8, below). Furthermore, in order to improve the cycle characteristics, a hydrophobic layer such as a silane compound is formed on a superficial layer of a silicon material (see, for example, Patent Document 9, below). Still furthermore, in order to improve the cycle characteristics, silicon oxide is used, and a graphite film is formed on a superficial layer thereof to impart electric conductivity (see, for example, Patent Document 10, below). In the Patent Document 10, regarding a shift value obtained from a Raman spectrum of the graphite film, broad peaks appear at 1330 cm−1 and 1580 cm−1, and an intensity ratio thereof I1330/I1580 is 1.5<I1330/I1580<3.
Furthermore, in order to improve the cycle characteristics, silicon oxide is used, and a graphite film is formed on a superficial layer thereof to impart the electric conductivity (see, for example, Patent Document 11, below). In this case, regarding a shift value obtained from a Raman spectrum of the graphite film, broad peaks appear at 1330 cm−1 and 1580 cm−1, and an intensity ratio thereof I1330/I1580 is 1.5<I1330/I1580<3.
Furthermore, in order to obtain high battery capacity and to improve cycle characteristics, particles having a silicon crystallite phase dispersed in silicon dioxide are used (see, for example, Patent Document 12, below). Still furthermore, in order to improve overcharge and overdischarge characteristics, silicon oxide in which an atomic ratio of silicon and oxygen is controlled to 1:y (0<y<2) is used (see, for example, Patent Document 13, below).
The silicon oxide used as a negative electrode material for secondary batteries like this can be produced according to a method where a silicon powder and a silicon dioxide powder that are raw materials are supplied into a reaction furnace, and are heated under inert gas or under reduced pressure to generate silicon oxide gas, and the silicon oxide gas is cooled to deposit on a surface of a substrate (see, for example, Patent Document 14, below).